The expression of the HSV1 glycoprotein C gene was studied by cloning the promoter for this gene next to the coding sequences for the bacterial enzyme, beta-galactosidase. The expression of beta-galactosidase activity was directed by the glycoprotein C gene promoter and the level of beta-galactosidase activity was used as a measure of the promoter activity. Mammalian cell lines were transfected with this plasmid construction or with an identical plasmid construction minus the promoter sequences and assayed for galactosidase activity at 48 hours. The plasmid without a promoter never showed galactosidase activity. The plasmid with the glycoprotein C promoter expressed galactosidase activity but only when the cells were also infected with HSV1. Beta-galactosidase was also made from this plasmid if, instead of being infected with HSV1, the cells were co-transfected with a plasmid recombinant containing two of the HSV1 immediate-early genes. This transient assay system will be useful for determining what sequences in the glycoprotein C promoter are important in regulation.